Manhole coating: an alternative renovation method?
Aug 03, 2007
For the renovation of manholes, coating methods are considered a competitive alternative to the renewal of the manhole. But are they really an alternative? So far there has hardly been evidence regarding limitations of use, influences on quality and the durability of the methods offered. The IKT has lightened the dark: in an extensive practical survey 42 manhole coatings were monitored and investigated at twelve sewage network operators.
Besides the renewal of the manhole lining- and coating methods can be employed for the renovation of large-area damages or leakage. A specific pressure to act is apparent for operators in North Rhine Westphalia (Germany): here the Selbstüberwachungsverordnung Kanal (SüwV Kan) (self-monitoring regulations for sewer systems) require the inspection and, if necessary, the rehabilitation of manholes. According to estimations by public sewage network operators, around twelve per cent of the approximately ten million manholes in Germany are basically suitable for rehabilitation with the coating method. The resulting possible market volume for coating methods of around 2.7 billion Euro is immense! Until today, however, there have been large uncertainties regarding the limitations of use of the offered method and the durability of the results of the renovation.
The IKT chose a very practically-oriented procedure to find out the essential influences on quality when carrying out coating measures. At the same time they intended to sound out the possibilities or limits of use of each methods. During the two years of the project the following working program was completed:
42 coating measures in the drainage networks of altogether twelve network operators were monitored. Extensive quality test were carried out over a period of several months. Grout coatings and coatings of polyurethane were used by considering all cleaning- and application methods that are relevant for the market. All renovations were carried out by specialty firms that had only received instructions regarding the coating material and application method. Comments on the renovation procedure were deliberately not made in order to be able to identify the routine working processes, the actual quality of the renovation as well as possible sources of errors and improvement potentials under realistic circumstances.
To complement these on-site measures, experiments were carried out in the IKT-laboratories and test facilities. In this way, for instance, the effects of intensive traffic loads on a coated manhole could be simulated or the bonding behaviour of coatings on water-saturated concrete surfaces could be investigated further.
Further questions raised from the results of the investigations and the laboratory experiments: What are the minimum adhesion tensile strengths of coating? What are the effects of alternative pre-treatment techniques? How does the external climate influence the hardening of grout coatings? How well can the transition to other materials can be created in the joint areas? These questions were followed in additional investigations.
The results of the research project show: the coating methods analysed can basically be helpful to recover the functional capability of brick-built and concrete manholes. Especially with difficult geometries and numerous inflows renovating manholes by coating seems to be an alternative to the new construction of the manhole.
To keep a sustainable subsurface and the pre-conditions for a good bond of the coating, first of all dirt, coverings and glaze as well as damaged material have to be removed from the manhole wall. For this purpose, usually high-pressure water cleaning is used.
Brick-built manholes with strong joint corrosion and concrete manholes with advanced concrete corrosion have a very uneven wall surface. Before these manholes can be coated by spray lining, the surface needs to be flattened manually with a pre-coating of mineral mortar first.
The laboratory experiments clearly show big differences between the single products offered on the market. Especially the sensitivity towards moisture differs: while one of the PU-products analysed reacted to a high level of moisture, another PU-product showed no visually noticeable reactions.
All polyurethane products analysed did not adhere well on moist surfaces. Especially on PU-coatings which were applied to soaked brickwork, the IKT testers measured only a slight bond with the surface. In exceptional cases also debonding was observed.
Generally, there is a difference between the requirements of a (pre-coating) mortar and those of a polyurethane. While, if possible, the mortar should be kept rather moist during hardening, for coating with polyurethane a dry environment is an advantage.
Clever inspection before the acceptance of bid!
In the phase of preparing the renovation the manhole to be renovated should be visually inspected in detail. Ideally, the inspection should be carried out with a high level of groundwater in order to be able to notice leakage in the manhole body.
Nearly all manholes showed severe damage in the lower area of the manhole body, in many cases the berms, channels and connections to the outgoing sewers also needed renovation.
Damaged branches inside the manhole wall are to be renovated before starting the coating. Step irons should be demounted before coating. After the coating is completed, the use of a ladder is particularly recommended as then the body of the coating has only to be broken through at a few spots.
Generally, the following applies: the present practice under the special conditions in manholes seems to have a strong impact on the results of the renovation. Here in particular:
- procedural weaknesses in the preparation of the surface,
- manufacturers not meeting the requirements and the essential regulations during the entire renovation process and
- the lacking control of the rehabilitation success
Note: Control the tidiness of the manhole wall.
Note: Check the material used
Note: Mortar: urge the observation of manufacturer requirements
Note: Finishing treatment is highly important
Principally, it can be observed that the measures suggested in guidelines and progress reports are only suitable to a limited extent for testing the quality of a manhole coating in a sustainable way and especially with adequate effort. The following measures for quality checks have turned out to be reasonable:
Note: Approval of the construction after several months
Note: leaktightness testing does often not suffice
Note: Question the adhesion tensile test
As the article shows, basic findings regarding the influence of quality and limits of application of coating measures are now available. In the present research project "manholes – monitoring, testing and renovation" further questions are to be answered. It aims at the comprehensive analysis of the entire manhole body and the corresponding operation processes. In laboratory and in-situ applications various renovation processes are analysed in comparison.
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Dipl.-Ing. René Puhl [IKT – Institute for Underground Infrastructure, Germany]
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